Golborne Vintage Radio

Some time ago I built the Radio Bygones Wobbulator from April/May RB (Issue 82, June July 2003). At the same time, I started building the accompanying RF probe which featured in the next issue - RB 83, both projects being by the same author/designer, (the aptly named!) RF Haigh. As with so many of my projects, that got put to one aide waiting to be finished, so I decided to get on with it this week, and having finished it, it doesn't work. Grrrr.

On the face of things, the RF Probe is simple enough, and as it's built on a small PCB, the scope for error is small, except perhaps in the switching or getting the FET or diodes the wrong way round.

The probe seems to work fine up to the junction of D1 & D2, which function as a voltage doubler. However, nothing comes out at the cathode of D2. If I poke a 1 MHz square wave into the probe tip, it's fine at the input to D1 - nothing at the output. I've replace D2 (with an OA90 as the OA47 I removed, which probably wasn't faulty, is somewhere in the carpet pile). I don't think the OA90 will make and difference to the proper functioning of the probe (when/if it ever does function!). I've also replaced R4 and C4, neither of which on removal seemed duff.

I've not heard of any corrections to the circuit, but am left wondering if there is a fault in the circuit or the values of R4/C4 which isn't apparent to me. (I did spot a fault in the Wobbulator layout as coils 3&4 had been transposed on the PCB layout diagram. I didn't discover that until I was testing it on my frequency counter and noted that the coverage of bands 3 & 4 were transposed.

Any thoughts anyone please before this ends up back in the 'round tuit' box?

What are you measuring the output with David?

Lawrence.

I'm not using it with the Wobbulator - I'm just injecting a 1 MHz square wave from a waveform generator to the probe, then observing the waveform and tracing it through to the junction of the cathode of D1 & the input to D2, up to which point the signal is fine. However, if I move the scope probe to the cathode of D2, there isn't a whisper of anything. Huh?

I assume your scope is switched to DC input when connected to the output of the probe?

Lawrence.

This'll be a bit easier to read:-
[attachment=9429]

- Joe

Have you buzzed the tracks on the PCB for open circuit and shorts?
I had a PCB with a couple of open circuits on it that was made for a very much more complicated magazine project back in the 1970s. I did find the faults. It is still working and I finally got the faster chips working a couple of years ago.

Thanks so much for your interest, helpful suggestions and encouragement guys. I've not had chance to look further into it but hope to do so tomorrow and will report back.

Thanks too to Joe for collating and tidying up the article, which might be of interest to others as a project as it isn't simply for the RB Wobbulator.

Night night!

I've re-read the article to ensure that I understand how (in theory) the RF probe is supposed to operate. Its aim is to take a low level RF signal and to amplify it to enable the response curve to be displayed on the 'scope screen. At the bottom of Page 27 of the article it shows two traces of what to expect at the output of the probe. I've done a few quick tests and have injected a 1 MHz signal and have shown the traces obtained below. In all cases the scope is set to 0.5V per division.

The first pic is at the input to the probe tip, ( about 0.75V P to p); the second is at the output (cathode) of D1 with the probe set to the 'low' setting; the third pic is with the probe in the high setting (about 3V p to p), which is about which indicates that the J310 amplifier stage is working. The fourth pic shows the scope trace with no connection to the probe to show the baseline I set the scope to. The final pic is at the output of the probe with the scope set to DC, which shows the trace lifted by about 3V from pic 1. (With the scope set to AC, there is no AC output - ie RF waveform - from the probe). This seems to indicate that D2 - rather than working with D1 as a voltage doubler to display low level RF signal inputs to the probe - is simply acting as a rectifier, rectifying the 1 Mz input to D2 to DC at its output. Thus, the probe will give a DC voltage output of an RF signal input, (as stated in the top para on the second column of page 27 of the article) but will not display an RF waveform as indicated by the two pics at the bottom of page 27.

I remain perplexed - I'm having a Donald Rumsfeld moment - it's a 'known unknown' !

Small project - I might just abandon it and build another, but it's often said that to do the same thing over again and to expect a different outcome is the first sign of insanity.

Confused, of Cottingham.

The two pictures on P27 are the response curves for the filters mentioned. They are similar in principle to the response curves I showed at the demo at Golborne.

The probe is effectively just a rectifier producing a dc voltage which is roughly proportional to the level of the RF input and looking at your pictures it would appear to be working correctly.

Keith

(25-11-2013, 09:34 AM)Yorkie Wrote: [ -> ]Thus, the probe will give a DC voltage output of an RF signal input, (as stated in the top para on the second column of page 27 of the article) but will not display an RF waveform as indicated by the two pics at the bottom of page 27.

If I'm reading this correct David are you expecting an RF waveform at the output, if so your not going to get one.

Lawrence. [EDIT: Whoops....Keith beat me to it, as he says, the waveforms look about right, the DC shift looks about right also taking into account the forward voltage drop of the detector, when you rig it up to to a receiver with a swept frequency input from your sweep generator don't connect the probe to the detector output of the receiver.]